Ammonia slip catalyst designed to be first in an scr system

ABSTRACT

Catalyst articles having an ammonia slip catalyst (ASC) comprising a blend of platinum on a support with low ammonia storage and a first SCR catalyst, and a second catalyst, such as a diesel oxidation catalyst, a diesel exotherm catalyst (DEC), a NOx absorber, a selective catalytic reduction/passive NOx adsorber (SCR/PNA), a cold-start catalyst (CSC) or a three-way catalyst (TWC) are disclosed. The catalyst articles can also contain one or two additional SCR catalysts. The catalysts can be present in one of various configurations. The catalytic articles are useful for selective catalytic reduction (SCR) of NOx in exhaust gases and in reducing the amount of ammonia slip. Methods of using the catalytic articles in an SCR process, where the amount of ammonia slip is reduced, are also described.

FIELD OF THE INVENTION

The invention relates to ammonia slip catalysts (ASC), articlescontaining ammonia slip catalysts and methods of manufacturing and usingsuch articles to reduce ammonia slip.

BACKGROUND OF THE INVENTION

Hydrocarbon combustion in diesel engines, stationary gas turbines, andother systems generates exhaust gas that must be treated to removenitrogen oxides (NOx), which comprises NO (nitric oxide) and NO₂(nitrogen dioxide), with NO being the majority of the NOx formed. NOx isknown to cause a number of health issues in people as well as causing anumber of detrimental environmental effects including the formation ofsmog and acid rain. To mitigate both the human and environmental impactfrom NO_(x) in exhaust gas, it is desirable to eliminate theseundesirable components, preferably by a process that does not generateother noxious or toxic substances.

Exhaust gas generated in lean-burn and diesel engines is generallyoxidative. NOx needs to be reduced selectively with a catalyst and areductant in a process known as selective catalytic reduction (SCR) thatconverts NOx into elemental nitrogen (N₂) and water. In an SCR process,a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, orurea, is added to an exhaust gas stream prior to the exhaust gascontacting the catalyst. The reductant is absorbed onto the catalyst andthe NO_(x) is reduced as the gases pass through or over the catalyzedsubstrate. In order to maximize the conversion of NOx, it is oftennecessary to add more than a stoichiometric amount of ammonia to the gasstream. However, release of the excess ammonia into the atmosphere wouldbe detrimental to the health of people and to the environment. Inaddition, ammonia is caustic, especially in its aqueous form.Condensation of ammonia and water in regions of the exhaust linedownstream of the exhaust catalysts can result in a corrosive mixturethat can damage the exhaust system. Therefore the release of ammonia inexhaust gas should be eliminated. In many conventional exhaust systems,an ammonia oxidation catalyst (also known as an ammonia slip catalyst or“ASC”) is installed downstream of the SCR catalyst to remove ammoniafrom the exhaust gas by converting it to nitrogen. The use of ammoniaslip catalysts can allow for NO_(x) conversions of greater than 90% overa typical diesel driving cycle.

It would be desirable to have a catalyst that provides for both NOxremoval by SCR and for selective ammonia conversion to nitrogen, whereammonia conversion occurs over a wide range of temperatures in avehicle's driving cycle, and minimal nitrogen oxide and nitrous oxidebyproducts are formed.

SUMMARY OF THE INVENTION

In a first aspect, the invention relates to a catalyst articlecomprising a substrate comprising an inlet end and an outlet end, afirst zone and a second zone, where the first zone comprises an ammoniaslip catalyst (ASC) comprising a platinum group metal and a first SCRcatalyst comprising a metal exchange molecular sieve, vanadium or a basemetal, and the second zone comprises a second catalyst selected from thegroup consisting of a diesel oxidation catalyst (DOC), a diesel exothermcatalyst (DEC), a catalyzed soot filter (CSF), a NOx absorber, aselective catalytic reduction/passive NOx adsorber (SCR/PNA), acold-start catalyst (CSC) or a three-way catalyst (TWC), where the firstzone is located upstream of the gas flow relative to the second zone.

In another aspect, the invention relates to exhaust systems comprising acatalytic article of the first aspect of the invention and a means forforming NH₃ in the exhaust gas.

In yet another aspect, the invention relates to a combustion sourcecomprising an exhaust system comprising a catalyst article of the firstaspect of the invention and a means for forming NH₃ in the exhaust gas

In still another aspect, the invention relates to a method of providingan exotherm in a catalyst where the method comprises contacting anexhaust gas comprising hydrocarbons with the catalyst of the firstaspect of the invention.

In another aspect, the invention relates to a method of reducing N₂Oformation from NH₃ in an exhaust gas, where the method comprisescontacting an exhaust gas comprising ammonia with a catalyst article ofthe first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a configuration in which the ASC is positioned in theexhaust gas flow before a DOC or other catalyst.

FIG. 2 depicts a configuration in which the ASC is a bi-layer and ispositioned in the exhaust gas flow before a DOC or other catalyst.Depictions of the bi-layers are also shown.

FIG. 3 depicts a configuration in which the ASC is a blend and ispositioned in the exhaust gas flow before a DOC or other catalyst.

FIG. 4 depicts a configuration in which an ASC is positioned in theexhaust gas flow before a filter and a SCR is placed after the filter,where the ASC can be a bi-layer or a blend and the filter is coated onnon-coated.

FIG. 5 depicts a configuration in which an ASC is positioned in theexhaust gas flow before and after a filter, where the ASC before thefilter is a bi-layer with the SCR covering both the inlet side and thetop of the ammonia oxidation catalyst, the ASC after the filter is abi-layer with the SCR covering the inlet side of the ammonia oxidationcatalyst the and the filter is coated on non-coated.

FIG. 6 depicts a configuration in which an ASC is positioned in theexhaust gas flow before and after a filter, where both ASCs are abi-layer with the SCR covering both the inlet side and the top of theammonia oxidation catalyst the and the filter is coated on non-coated.

FIG. 7 depicts a configuration in which the ASC is positioned in theexhaust gas flow before a DOC or other catalyst and the ASC is abi-layer with the ammonia slip catalyst on the inlet side of the SCR andthe SCR is between the DOC or other catalyst.

FIG. 8 depicts a configuration in which the ASC is positioned in theexhaust gas flow before a DOC or other catalyst and the ASC is abi-layer with the ammonia slip catalyst on the inlet side of the SCR anda portion of the SCR covers the top of the ammonia oxidation catalystand the SCR is between the DOC or other catalyst.

FIG. 9 depicts a configuration in which the ASC is positioned in theexhaust gas flow before an SCR catalyst that is upstream of a DOC orother catalyst, where the ASC is a bi-layer with the SCR catalyst beforethe ammonia oxidation catalyst. In other configurations, not shown, aportion of either the SCR catalyst in the ASC or the SCR between the ASCand the DOC or other catalyst, can cover the top of the ammoniaoxidation layer.

FIG. 10 is a graph showing NH₃ conversion, N₂O selectivity and NOxselectivity using fresh catalysts.

FIG. 11 is a graph showing NH₃ conversion, N₂O selectivity and NOxselectivity using aged catalysts.

FIG. 12 is a graph showing NO conversion using fresh catalysts.

FIG. 13 is a graph showing NO conversion using aged catalysts.

FIG. 14 is a graph showing CO conversion using fresh catalysts.

FIG. 15 is a graph showing CO conversion using aged catalysts.

FIG. 16 is a graph showing hydrocarbon (HC) conversion using a freshreference catalyst.

FIG. 17 is a graph showing hydrocarbon (HC) conversion using a freshcatalyst having a 1:5 Pt:Pd ratio.

FIG. 18 is a graph showing hydrocarbon (HC) conversion using a freshcatalyst having a 2:1 Pt:Pd ratio.

FIG. 19 is a graph showing hydrocarbon (HC) conversion using an agedreference catalyst.

FIG. 20 is a graph showing hydrocarbon (HC) conversion using an agedcatalyst having a 1:5 Pt:Pd ratio.

FIG. 21 is a graph showing hydrocarbon (HC) conversion using an agedcatalyst having a 2:1 Pt:Pd ratio.

FIG. 22 is a graph showing the temperature at various points in anexhaust system containing a reference catalyst with Pt as the only PGM.

FIG. 23 is a graph showing the temperature at various points in anexhaust system containing a reference catalyst with Pt and Pd in a 1:5loading.

FIG. 24 is a graph showing the temperature at various points in anexhaust system containing a reference catalyst with Pt and Pd in a 2:1loading.

DETAILED DESCRIPTION OF THE INVENTION

As used in this specification and the appended claims, the singularforms “a”, “an” and “the” include plural referents unless the contextclearly indicates otherwise. Thus, for example, reference to “acatalyst” includes a mixture of two or more catalysts, and the like.

The term “ammonia slip”, means the amount of unreacted ammonia thatpasses through the SCR catalyst.

The term “support” means the material to which a catalyst is fixed.

The term “a support with low ammonia storage” means a support thatstores less than 0.001 mmol NH₃ per m³ of support. The support with lowammonia storage is preferably a molecular sieve or zeolite having aframework type selected from the group consisting of AEI, ANA, ATS, BEA,CDO, CFI, CHA, CON, DDR, ERI, FAU, FER, GON, IFR, IFW, IFY, IHW, IMF,IRN, IRY, ISV, ITE, ITG, ITN, ITR, ITW, IWR, IWS, IWV, IWW, JOZ, LTA,LTF, MEL, MEP, MFI, MRE, MSE, MTF, MTN, MTT, MTW, MVY, MWW, NON, NSI,RRO, RSN, RTE, RTH, RUT, RWR, SEW, SFE, SFF, SFG, SFH, SFN, SFS, SFV,SGT, SOD, SSF, SSO, SSY, STF, STO, STT, SVR, SVV, TON, TUN, UOS, UOV,UTL, UWY, VET, VNI. More preferably, the molecular sieve or zeolite hasa framework type selected from the group consisting of BEA, CDO, CON,FAU, MEL, MFI and MWW, even more preferably the framework type isselected from the group consisting of BEA and MFI.

The term “calcine”, or “calcination”, means heating the material in airor oxygen. This definition is consistent with the IUPAC definition ofcalcination. (IUPAC. Compendium of Chemical Terminology, 2nd ed. (the“Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. BlackwellScientific Publications, Oxford (1997). XML on-line corrected version:http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B.Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.doi:10.1351/goldbook.) Calcination is performed to decompose a metalsalt and promote the exchange of metal ions within the catalyst and alsoto adhere the catalyst to a substrate. The temperatures used incalcination depend upon the components in the material to be calcinedand generally are between about 400° C. to about 900° C. forapproximately 1 to 8 hours. In some cases, calcination can be performedup to a temperature of about 1200° C. In applications involving theprocesses described herein, calcinations are generally performed attemperatures from about 400° C. to about 700° C. for approximately 1 to8 hours, preferably at temperatures from about 400° C. to about 650° C.for approximately 1 to 4 hours.

The term “about” means approximately and refers to a range that isoptionally ±25%, preferably ±10%, more preferably, ±5%, or mostpreferably ±1% of the value with which the term is associated.

When a range, or ranges, for various numerical elements are provided,the range, or ranges, can include the values, unless otherwisespecified.

The term “N₂ selectivity” means the percent conversion of ammonia intonitrogen.

The terms “diesel oxidation catalyst” (DOC), “diesel exotherm catalyst”(DEC), “NOx absorber”, “SCR/PNA” (selective catalytic reduction/passiveNOx adsorber), “cold-start catalyst” (CSC) and “three-way catalyst”(TWC) are well known terms in the art used to describe various types ofcatalysts used to treat exhaust gases from combustion processes.

The term “platinum group metal” or “PGM” refers to platinum, palladium,ruthenium, rhodium, osmium and iridium. The platinum group metals arepreferably platinum, palladium, ruthenium or rhodium.

The terms “downstream” and “upstream” describe the orientation of acatalyst or substrate where the flow of exhaust gas is from the inletend to the outlet end of the substrate or article.

In the first aspect of the invention, a catalyst article comprises asubstrate comprising an inlet end and an outlet end, a first zone and asecond zone, where the first zone comprises an ammonia slip catalyst(ASC) comprising a platinum group metal and a first SCR catalystcomprising a metal exchange molecular sieve, vanadium or a base metal,and the second zone comprises a second catalyst selected from the groupconsisting of a diesel oxidation catalyst (DOC), a diesel exothermcatalyst (DEC), a catalyzed soot filter (CSF), a NOx absorber, aselective catalytic reduction/passive NOx adsorber (SCR/PNA), acold-start catalyst (CSC) or a three-way catalyst (TWC), where the firstzone is located upstream of the gas flow relative to the second zone.

The ammonia slip catalyst comprises an ammonia oxidation catalystcomprising a platinum group metal and a first SCR catalyst comprising ametal exchange molecular sieve, vanadium, a base metal, a base metaloxide or a mixed oxide. The PGM catalyst and the SCR catalyst can bepresent in one of four configurations: three types of bi-layerconfigurations and a blend. The first bi-layer configuration has theammonia oxidation catalyst in a bottom layer and an SCR catalyst in thetop layer. The second bi-layer configuration has the SCR catalyst in alayer before a layer comprising the ammonia oxidation catalyst. Thethird bi-layer configuration has the SCR catalyst in a layer before alayer comprising the ammonia oxidation catalyst and a portion of the SCRcatalyst is also present in a top layer over the ammonia oxidationcatalyst. These bi-layer configurations can be depicted as:

$\begin{matrix}\frac{S\; C\; R\mspace{14mu} {catalyst}}{{ammonia}\mspace{14mu} {oxidation}\mspace{14mu} {catalyst}} & (1) \\{{S\; C\; R\mspace{14mu} {catalyst}}{{ammonia}\mspace{14mu} {oxidation}\mspace{14mu} {catalyst}}} & (2) \\{{S\; C\; R\mspace{14mu} {catalyst}}\frac{S\; C\; R\mspace{14mu} {catalyst}}{{ammonia}\mspace{14mu} {oxidation}\mspace{14mu} {catalyst}}} & (3)\end{matrix}$

In each of these configurations, the layer with the SCR catalyst canalso include a platinum group metal, preferably platinum or palladium,more preferably palladium. The ammonia slip catalyst can comprises abottom layer comprising a platinum group metal and a top layercomprising the first SCR catalyst located over the bottom layer.

In the fourth configuration, the ammonia slip catalyst comprises a blendof a platinum group metal (PGM) on a support with low ammonia storageand a first SCR catalyst. Preferably, the platinum group metal comprisesplatinum, palladium or a mixture thereof. The blend can further comprisePd, Nb—Ce—Zr or Nb on MnO₂.

When the article comprises two or more ASCs, the ammonia oxidationcatalyst and the SCR catalyst in the ASCs can be the same or different.

The first zone and the second zone can be located on the same substratewhere the first zone is located on the inlet side of the substrate andthe second zone is located on the outlet side of the substrate. Thecatalyst article can further comprise a second substrate, where thefirst zone is located on a first substrate and the second zone islocated on the second substrate and the first substrate is locatedupstream of the second substrate. The catalyst article can comprise afirst piece and a second piece, where the first zone is located in thefirst piece and the second zone is located in the second piece and thefirst piece is located upstream of the second piece. The catalystarticle can comprise a first piece and a second piece, where a portionof the first zone is located in the first piece and the remainder of thefirst zone and the second zone is located in the second piece and thefirst piece is located upstream of the second piece. The catalystarticle can comprise a three first piece and a second piece, where thefirst zone is located in the first piece and the remainder of the firstzone and the second zone is located in the second piece and the firstpiece is located upstream of the second piece. In each of thesecombinations of pieces, additional catalysts can also be place in thesecond piece or on additional pieces after the second piece.

The amount of PGM in the ASC can vary depending upon the composition ofthe catalyst article. For example, the PGM can be present at levels offrom about 0.1 g/ft³ to about 5 g/ft³, preferably from about 0.1 g/ft³to about 1 g/ft³. In some configurations, when it is desirable togenerate an exotherm, PGM can be present at levels of from about 1 g/ft³to about 20 g/ft³, preferably from about 5 g/ft³ to about 10 g/ft³. Insome configurations the PGM is Pt, Pd or a combination of platinum andpalladium. When both Pt and Pd are used, the ratio of Pt:Pd can bebetween about 10:1 and 1:100, preferably from between about 5:1 and1:10.

The ammonia oxidation catalyst can comprise platinum on a support withlow ammonia storage. The support with low ammonia storage can be asiliceous support. The siliceous support can comprise a silica or azeolite with a silica-to-alumina ratio of at least one of: (a) at least100, (b) at least 200, (c) at least 250, (d) at least 300, (e) at least400, (f) at least 500, (g) at least 750 and (h) at least 1000. Thesiliceous support can comprise a molecular sieve having a BEA, CDO, CON,FAU, MEL, MFI or MWW Framework Type. The ratio of the amount of the SCRcatalyst to the amount of platinum on the support with low ammoniastorage can be in the range of 0:1 to 300:1, preferably 3:1 to 300:1,more preferably 7:1 to 100:1 and most preferably 10:1 to 50:1, includingeach of the end-points in the ratio, based on the weight of thesecomponents.

The second zone can comprise a blend of an oxidation catalyst and asecond SCR catalyst.

The catalytic article can further comprise a second SCR catalyst, wherethe second SCR catalyst is located either upstream of the ammonia slipcatalyst or between the ammonia slip catalyst and the second catalyst. Aportion of the second SCR catalyst can at least partially overlap theammonia slip catalyst.

The catalytic article can comprise a second SCR catalyst, where thesecond SCR catalyst is located between the ammonia oxidation catalystand the second catalyst (i.e. downstream of an ASC and upstream of adiesel oxidation catalyst (DOC), a diesel exotherm catalyst (DEC), acatalyzed soot filter (CSF), a NOx absorber, a selective catalyticreduction/passive NOx adsorber (SCR/PNA), a cold-start catalyst (CSC) ora three-way catalyst (TWC).)

SCR Catalysts

The catalyst article can comprise one or more SCR catalysts. The SCRcatalyst present in each of the ammonia slip catalyst is called a firstSCR catalyst. This SCR catalyst is present in the ASC as part of abi-layer or in a blend with Pt on a support with low ammonia storage.The first SCR catalyst can be a metal exchange molecular sieve, vanadiumor a base metal. The first SCR catalyst is preferably a Cu—SCR catalyst,a Fe—SCR catalyst or a mixed oxide, more preferably a Cu—SCR catalyst ora mixed oxide. The Cu—SCR catalyst comprises copper and a molecularsieve. The Fe—SCR catalyst comprises iron and a molecular sieve.Molecular sieves are further described below. Copper or iron can belocated within the framework of the molecular sieve and/or inextra-framework (exchangeable) sites within the molecular sieve. Thefirst SCR catalyst is preferably a Cu—SCR catalyst comprising copper anda small pore molecular sieve or an Fe—SCR catalyst comprising iron and asmall pore molecular sieve. The small pore molecular sieve can be analuminosilicate, an aluminophosphate (AlPO), a silico-aluminophosphate(SAPO), or mixtures thereof. The small pore molecular sieve can beselected from the group of Framework Types consisting of ACO, AEI, AEN,AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI,ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI,RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG, and ZON, andmixtures and/or intergrowths thereof. Preferably, the small poremolecular sieve can be selected from the group of Framework Typesconsisting of CHA, LEV, AEI, AFX, ERI, SFW, KFI, DDR and ITE. The ratioof the amount of the first SCR catalyst to the amount of platinum on thesupport with low ammonia storage can be in the range of at least one of:(a) 0:1 to 300:1, (b) 3:1 to 300:1, (c) 7:1 to 100:1; and (d) 10:1 to50:1, inclusive, based on the weight of these components. Platinum canbe present from at least one of: (a) 0.01-0.3 wt. %, (b) 0.03-0.2 wt. %,(c) 0.05-0.17 wt. %, and (d) 0.07-0.15 wt. %, inclusive, relative to theweight of the support of platinum+the weight of platinum+the weight ofthe first SCR catalyst in the blend.

A second SCR catalyst can be a base metal, an oxide of a base metal, amolecular sieve, a metal exchanged molecular sieve or a mixture thereof.The base metal can be selected from the group consisting of vanadium(V), molybdenum (Mo), tungsten (W), chromium (Cr), cerium (Ce),manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), andzirconium (Zr) and mixtures thereof. SCR compositions consisting ofvanadium supported on a refractory metal oxide such as alumina, silica,zirconia, titania, ceria and combinations thereof are well known andwidely used commercially in mobile applications. Typical compositionsare described in U.S. Pat. Nos. 4,010,238 and 4,085,193, the entirecontents of which are incorporated herein by reference. Compositionsused commercially, especially in mobile applications, comprise TiO₂ onto which WO₃ and V₂O₅ have been dispersed at concentrations ranging from5 to 20 wt. % and 0.5 to 6 wt. %, respectively. The second SCR catalystcan comprise Nb—Ce—Zr or Nb on MnO₂. These catalysts may contain otherinorganic materials such as SiO₂ and ZrO₂ acting as binders andpromoters.

When the SCR catalyst is a base metal, the catalyst article can furthercomprise at least one base metal promoter. As used herein, a “promoter”is understood to mean a substance that when added into a catalyst,increases the activity of the catalyst. The base metal promoter can bein the form of a metal, an oxide of the metal, or a mixture thereof. Theat least one base metal catalyst promoter may be selected from neodymium(Nd), barium (Ba), cerium (Ce), lanthanum (La), praseodymium (Pr),magnesium (Mg), calcium (Ca), manganese (Mn), zinc (Zn), niobium (Nb),zirconium (Zr), molybdenum (Mo), tin (Sn), tantalum (Ta), strontium (Sr)and oxides thereof. The at least one base metal catalyst promoter canpreferably be MnO₂, Mn₂O₃, Fe₂O₃, SnO₂, CuO, CoO, CeO₂ and mixturesthereof. The at least one base metal catalyst promoter may be added tothe catalyst in the form of a salt in an aqueous solution, such as anitrate or an acetate. The at least one base metal catalyst promoter andat least one base metal catalyst, e.g., copper, may be impregnated froman aqueous solution onto the oxide support material(s), may be addedinto a washcoat comprising the oxide support material(s), or may beimpregnated into a support previously coated with the washcoat.

The SCR catalyst can comprises a molecular sieve or a metal exchangedmolecular sieve. As is used herein “molecular sieve” is understood tomean a metastable material containing tiny pores of a precise anduniform size that may be used as an adsorbent for gases or liquids. Themolecules which are small enough to pass through the pores are adsorbedwhile the larger molecules are not. The molecular sieve can be azeolitic molecular sieve, a non-zeolitic molecular sieve, or a mixturethereof.

A zeolitic molecular sieve is a microporous aluminosilicate having anyone of the framework structures listed in the Database of ZeoliteStructures published by the International Zeolite Association (IZA). Theframework structures include, but are not limited to those of the CHA,FAU, BEA, MFI, MOR types. Non-limiting examples of zeolites having thesestructures include chabazite, faujasite, zeolite Y, ultrastable zeoliteY, beta zeolite, mordenite, silicalite, zeolite X, and ZSM-5.Aluminosilicate zeolites can have a silica/alumina molar ratio (SAR)defined as SiO₂/Al₂O₃) from at least about 5, preferably at least about20, with useful ranges of from about 10 to 200.

Any of the SCR catalysts can comprise a small pore, a medium pore or alarge pore molecular sieve, or combinations thereof. A “small poremolecular sieve” is a molecular sieve containing a maximum ring size of8 tetrahedral atoms. A “medium pore molecular sieve” is a molecularsieve containing a maximum ring size of 10 tetrahedral atoms. A “largepore molecular sieve” is a molecular sieve having a maximum ring size of12 tetrahedral atoms. The second SCR catalyst can comprise a small poremolecular sieve selected from the group consisting of aluminosilicatemolecular sieves, metal-substituted aluminosilicate molecular sieves,aluminophosphate (AlPO) molecular sieves, metal-substitutedaluminophosphate (MeAlPO) molecular sieves, silico-aluminophosphate(SAPO) molecular sieves, and metal substituted silico-aluminophosphate(MeAPSO) molecular sieves, and mixtures thereof.

Any of the SCR catalysts can comprise a small pore molecular sieveselected from the group of Framework Types consisting of ACO, AEI, AEN,AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI,ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI,RHO, RTH, SAT, SAV, SFW, SIV, THO, TSC, UEI, UFI, VNI, YUG, and ZON, andmixtures and/or intergrowths thereof. Preferably the small poremolecular sieve is selected from the group of Framework Types consistingof AEI, AFX, CHA, DDR, ERI, ITE, KFI, LEV and SFW.

Any of the SCR catalysts can comprise a medium pore molecular sieveselected from the group of Framework Types consisting of AEL, AFO, AHT,BOF, BOZ, CGF, CGS, CHI, DAC, EUO, FER, HEU, IMF, ITH, ITR, JRY, JSR,JST, LAU, LOV, MEL, MFI, MFS, MRE, MTT, MVY, MWW, NAB, NAT, NES, OBW,PAR, PCR, PON, PUN, RRO, RSN, SFF, SFG, STF, STI, STT, STW, SVR, SZR,TER, TON, TUN, UOS, VSV, WEI, and WEN, and mixtures and/or intergrowthsthereof. Preferably, the medium pore molecular sieve selected from thegroup of Framework Types consisting of FER, MFI, and STT.

Any of the SCR catalysts can comprise a large pore molecular sieveselected from the group of Framework Types consisting of AFI, AFR, AFS,AFY, ASV, ATO, ATS, BEA, BEC, BOG, BPH, BSV, CAN, CON, CZP, DFO, EMT,EON, EZT, FAU, GME, GON, IFR, ISV, ITG, IWR, IWS, IWV, IWW, JSR, LTF,LTL, MAZ, MEI, MOR, MOZ, MSE, MTW, NPO, OFF, OKO, OSI, RON, RWY, SAF,SAO, SBE, SBS, SBT, SEW, SFE, SFO, SFS, SFV, SOF, SOS, STO, SSF, SSY,USI, UWY, and VET, and mixtures and/or intergrowths thereof. Preferably,the large pore molecular sieve is selected from the group of FrameworkTypes consisting of BEA, MOR and OFF.

A metal exchanged molecular sieve can have at least one metal from oneof the groups VB, VIB, VIIB, VIIIB, IB, or IIB of the periodic tabledeposited onto extra-framework sites on the external surface or withinthe channels, cavities, or cages of the molecular sieves. Metals may bein one of several forms, including, but not limited to, zero valentmetal atoms or clusters, isolated cations, mononuclear or polynuclearoxycations, or as extended metal oxides. Preferably, the metals can beiron, copper, and mixtures or combinations thereof.

The metal can be combined with the zeolite using a mixture or a solutionof the metal precursor in a suitable solvent. The term “metal precursor”means any compound or complex that can be dispersed on the zeolite togive a catalytically-active metal component. Preferably the solvent iswater due to both economics and environmental aspects of using othersolvents. When copper, a preferred metal is used, suitable complexes orcompounds include, but are not limited to, anhydrous and hydrated coppersulfate, copper nitrate, copper acetate, copper acetylacetonate, copperoxide, copper hydroxide, and salts of copper amines (e.g. [Cu(NH₃)₄]²⁺).This invention is not restricted to metal precursors of a particulartype, composition, or purity. The molecular sieve can be added to thesolution of the metal component to form a suspension, which is thenallowed to react so that the metal component is distributed on thezeolite. The metal can be distributed in the pore channels as well as onthe outer surface of the molecular sieve. The metal can be distributedin ionic form or as a metal oxide. For example, copper may bedistributed as copper (II) ions, copper (I) ions, or as copper oxide.The molecular sieve containing the metal can be separated from theliquid phase of the suspension, washed, and dried. The resultingmetal-containing molecular sieve can then be calcined to fix the metalin the molecular sieve. Preferably, one or more SCR catalysts comprise aCu—SCR, an Fe—SCR, vanadium, a mixed oxide, promoted Ce—Zr or promotedMnO₂. A Cu—SCR catalyst comprises copper and a molecular sieve and anFe—SCR catalyst comprises iron and a molecular sieve. Preferably themolecular sieve is an aluminosilicate, an aluminophosphate (AlPO), asilico-aluminophosphate (SAPO), or mixtures thereof. When the second SCRcatalyst is between the ammonia slip catalyst and the second catalyst,the second SCR preferably comprises promoted Ce—Zr or promoted MnO₂.

A metal exchanged molecular sieve can contain in the range of about0.10% and about 10% by weight of a group VB, VIB, VIIB, VIIIB, IB, orIIB metal located on extra framework sites on the external surface orwithin the channels, cavities, or cages of the molecular sieve.Preferably, the extra framework metal can be present in an amount of inthe range of about 0.2% and about 5% by weight.

The metal exchanged molecular sieve can be a copper (Cu) or iron (Fe)supported molecular sieve having from about 0.1 wt. % to about 20 wt. %copper or iron of the total weight of the catalyst. More preferablycopper or iron is present from a about 0.5 wt. % to about 15 wt. % ofthe total weight of the catalyst. Most preferably copper or iron ispresent from about 1 wt. % to about 9 wt. % of the total weight of thecatalyst.

The catalysts described herein can be used in the SCR treatment ofexhaust gases from various engines. One of the properties of a catalystcomprising a blend of platinum on a siliceous support with a first SCRcatalyst, where the first SCR catalyst is a Cu—SCR or Fe—SCR catalyst,is that it can provide an improvement in N₂ yield from ammonia at atemperature from about 250° C. to about 350° C. compared to a catalystcomprising a comparable formulation in which the first SCR catalyst ispresent as a first layer and platinum is supported on a layer thatstores ammonia is present in a second coating and gas comprising NH₃passes through the first layer before passing through the secondcoating. Another property of a catalyst comprising a blend of platinumon a support with low ammonia storage with a first SCR catalyst, wherethe first SCR catalyst is a Cu—SCR catalyst or an Fe—SCR catalyst, isthat it can provide reduced N₂O formation from NH₃ compared to acatalyst comprising a comparable formulation in which the first SCRcatalyst is present as a first layer and platinum supported on a supportthat stores ammonia is present in a second coating and gas comprisingNH₃ passes through the first layer before passing through the secondcoating.

In a first configuration, a catalyst article comprises a substratehaving an inlet and an outlet, a first zone comprising an ammonia slipcatalyst (ASC) comprising (a) an ammonia oxidation catalyst comprising aPGM and a first SCR catalyst and a second zone comprising a dieseloxidation catalyst, a diesel exotherm catalyst (DEC), a NOx absorber, aselective catalytic reduction/passive NOx adsorber (SCR/PNA), acold-start catalyst (CSC) or a three-way catalyst (TWC), where the firstzone is located on the inlet side of the substrate and the second zoneis located on the outlet side of the substrate. FIG. 1 depicts aconfiguration in which the ASC is positioned at the inlet of the articlewithin the exhaust gas flow and the DOC or one of the other catalysts ispositioned at the outlet of the article. The ammonia slip catalyst canbe present in one of three Bi-layer configurations, as shown in FIG. 2.The ammonia slip catalyst can also be present as a blend of a platinumgroup metal (PGM) on a support with low ammonia storage and a first SCRcatalyst, where the platinum group metal preferably comprises platinum,palladium or a mixture thereof, as shown in FIG. 3.

In another configuration, a catalyst article comprises a substratehaving an inlet and an outlet, a first zone comprising an ammonia slipcatalyst (ASC) comprising (a) an ammonia oxidation catalyst comprising aPGM and a first SCR catalyst, a second zone comprising a coated oruncoated filter, followed by an SCR catalyst, as shown in FIG. 4. TheASC can be present as a bi-layer or a blend, as described above. FIG. 5shows a configuration in which the ASC in the first zone is configuredas a bi-layer in which the SCR catalyst covers the inlet side and thetop of the ammonia oxidation catalyst and the ASC at the outlet isconfigured as a bi-layer in which the SCR catalyst covers the inlet sideof the ammonia oxidation catalyst. FIG. 6 shows a configuration in whichboth ASCs are configured as a bi-layer in which the SCR catalyst coversthe inlet side and the top of the ammonia oxidation catalyst and the ASCat the outlet is configured as a bi-layer in which the SCR catalystcovers the inlet side of the ammonia oxidation catalyst.

FIG. 7 depicts a configuration in which the ASC is positioned in theexhaust gas flow before a DOC or another catalyst and the ASC is abi-layer with the ammonia oxidation catalyst on the inlet side of theSCR and the SCR is between the DOC or another catalyst.

FIG. 8 depicts a configuration in which the ASC is positioned in theexhaust gas flow before a DOC or other catalyst and the ASC is abi-layer with the ammonia slip catalyst on the inlet side of the SCR anda portion of the SCR covers the top of the ammonia oxidation catalystand the SCR is between the DOC or other catalyst. Preferably the secondSCR catalyst completely covers the first layer comprising a blend ofplatinum on a support with low ammonia storage with the first SCRcatalyst.

FIG. 9 depicts a configuration in which the ASC is positioned in theexhaust gas flow before a second SCR catalyst that is upstream of a DOCor other catalyst, where the ASC is a bi-layer with the SCR catalystbefore the ammonia oxidation catalyst. In other configurations, notshown, a portion of either the SCR catalyst in the ASC or the SCRbetween the ASC can cover the top of the ammonia oxidation layer.

In another configuration, the catalyst article comprises only oneammonia slip catalyst (ASC) and either (a) the second zone does notcomprise a diesel oxidation catalyst (DOC) or (b) the DOC is not locatedadjacent to, and downstream of, the ammonia slip catalyst.

In another configuration, the catalyst article further comprises a thirdzone comprising an ASC catalyst, where the third zone is located betweenthe first zone and the second zone. The PGM in the first zone cancomprise palladium in an amount sufficient to generate an exotherm.

In each of the above configurations each of the zones can be located onthe same substrate or there can be two or more substrates with one ormore zones on each substrate. In an exhaust system, when two or moresubstrates are used, one or more substrates can be located in a singlehousing or casing or in different housings or casings.

The substrate for the catalyst may be any material typically used forpreparing automotive catalysts that comprises a flow-through or filterstructure, such as a honeycomb structure, an extruded support, ametallic substrate, or a SCRF. Preferably the substrate has a pluralityof fine, parallel gas flow passages extending from an inlet to an outletface of the substrate, such that passages are open to fluid flow. Suchmonolithic carriers may contain up to about 700 or more flow passages(or “cells”) per square inch of cross section, although far fewer may beused. For example, the carrier may have from about 7 to 600, moreusually from about 100 to 400, cells per square inch (“cpsi”). Thepassages, which are essentially straight paths from their fluid inlet totheir fluid outlet, are defined by walls onto which the SCR catalyst iscoated as a “washcoat” so that the gases flowing through the passagescontact the catalytic material. The flow passages of the monolithicsubstrate are thin-walled channels which can be of any suitablecross-sectional shape such as trapezoidal, rectangular, square,triangular, sinusoidal, hexagonal, oval, circular, etc. The invention isnot limited to a particular substrate type, material, or geometry.

Ceramic substrates may be made of any suitable refractory material, suchas cordierite, cordierite-α alumina, α-alumina, silicon carbide, siliconnitride, zirconia, mullite, spodumene, alumina-silica magnesia,zirconium silicate, sillimanite, magnesium silicates, zircon, petalite,aluminosilicates and mixtures thereof.

Wall flow substrates may also be formed of ceramic fiber compositematerials, such as those formed from cordierite and silicon carbide.Such materials are able to withstand the environment, particularly hightemperatures, encountered in treating the exhaust streams.

The substrates can be a high porosity substrate. The term “high porositysubstrate” refers to a substrate having a porosity of between about 40%and about 80%. The high porosity substrate can have a porositypreferably of at least about 45%, more preferably of at least about 50%.The high porosity substrate can have a porosity preferably of less thanabout 75%, more preferably of less than about 70%. The term porosity, asused herein, refers to the total porosity, preferably as measured withmercury porosimetry.

Preferably, the substrate can be cordierite, a high porosity cordierite,a metallic substrate, an extruded SCR, a filter or an SCRF.

A washcoat comprising a blend of platinum on a siliceous support and afirst SCR catalyst, where the first SCR catalyst is preferably a Cu—SCRcatalyst or an Fe—SCR catalyst, can be applied to the inlet side of thesubstrate using a method known in the art. After application of thewashcoat, the composition can be dried and optionally calcined. When thecomposition comprises a second SCR, the second SCR can be applied in aseparate washcoat to either a dried or calcined article having thebottom layer, as described above. After the second washcoat is applied,it can be dried and calcined.

The substrate with the platinum containing layer can be dried andcalcined at a temperature within the range of 300° C. to 1200° C.,preferably 400° C. to 700° C., and more preferably 450° C. to 650° C.The calcination is preferably done under dry conditions, but it can alsobe performed hydrothermally, i.e., in the presence of some moisturecontent. Calcination can be performed for a time of between about 30minutes and about 4 hours, preferably between about 30 minutes and about2 hours, more preferably between about 30 minutes and about 1 hour.

An exhaust system can comprise a catalyst article of the first aspect ofthe invention and a means of introducing NH₃ into the exhaust gas or forforming NH₃ in the exhaust gas. In an exhaust system, when two or moresubstrates are used, one or more substrates can be located in a singlehousing or casing or in different housings or casings. The exhaustsystem can further comprise a catalysed soot filter (CSF). The catalysedsoot filter comprises a high PGM loading in the front of the filter. Ahigh PGM loading means a loading of from about 5 g/ft³ to about 20 g/ft³(preferably about at least 5 g/Wt in heavy duty diesel engines and fromabout 10 g/ft³ to about 20 g/ft³ in light duty diesel engines) in aboutthe front 5-50 mm of the filter.

An engine can comprise an exhaust system comprising a catalyst articleof the first aspect of the invention and a means of introducing NH₃ intothe exhaust gas or forming NH₃ in the exhaust gas. The engine can be adiesel engine on a vehicle, a diesel engine on a stationary source, or adiesel engine on a vessel, such as a ship.

In another aspect of the invention, a method of providing an exotherm ina catalyst comprises contacting an exhaust gas comprising a combustiblegas, such a hydrocarbons, carbon monoxide (CO) or hydrogen, with thecatalyst of the first aspect of the invention.

In another aspect of the invention, a method of reducing N₂O formationfrom NH₃ in an exhaust gas comprises contacting an exhaust gascomprising ammonia with the catalytic article of the first aspect of theinvention.

The following examples merely illustrate the invention; the skilledperson will recognize many variations that are within the spirit of theinvention and scope of the claims.

Examples Example 1

Catalyst articles were prepared on a cordierite substrate (400 cpsi) byfirst placing a washcoat comprising a PGM on alumina on the substrate toform a bottom layer, then drying the washcoat. A top layer was placed onthe bottom layer by applying a washcoat comprising copper chabazite(Cu-CHA) (120 g/ft³ Cu), then drying the top layer. After the top layerhad dried, the article was calcined.

A reference catalyst article was prepared containing only platinum asthe PGM at a loading of 3 g/ft³. A sample comprising Pt and Pd as thePGM was prepared with a total PGM loading of 18 g/ft³, with a Pt:Pd atratio of 1:5. A sample comprising Pt and Pd as the PGM was prepared witha total PGM loading of 18 g/ft³, with a Pt:Pd at ratio of 2:1.

The samples were tested fresh and after hydrothermal ageing at 580° C.for 100 hours.

1″×1″ core of the samples first had N₂ gas passed over them as thetemperature increased from room temperature to 150° C. Then gascontaining NH₃=500 ppm, CO₂=4.5%, H₂O=5%, CO=200 ppm, 02=12%, with thebalance being N₂, was over the samples at SV=150000 h⁻¹, while thetemperature increased from 150° C. to 500° C. at a rate of 10°C./minute. The concentrations of NH₃, NOx, N₂O, CO and CO₂ were measuredby FTIR at the outlet from the system.

FIGS. 10 and 11 show NH₃ conversion, N₂O selectivity and NOx selectivityfrom the three samples from 200° C. to 500° C. in fresh and agedsamples. The catalyst with Pt and Pd in a 2:1 ratio provided better lowtemperature NH₃ conversion than the reference with only Pt, while thecatalyst with Pt and Pd in a 1:5 ratio provided less NH₃ conversionbelow about 350° C. The catalyst with Pt and Pd in a 2:1 ratio providedhigher N₂O selectivity than either the reference with only Pt or thecatalyst with Pt and Pd in a 1:5 ratio. The three catalysts providedsimilar NOx selectivity. These results apply to both fresh and agedsamples.

FIGS. 12 and 13 show NO conversions from the three samples from 150° C.to 500° C. in fresh and aged samples. Again, fresh and aged samplesprovided similar results, with the 2:1 mixture of Pt:Pd providingconversion equal to or greater than the reference with only Pt as thePGM.

FIGS. 14 and 15 show CO conversions from the three samples from 150° C.to 500° C. in fresh and aged samples. Again, fresh and aged samplesprovided similar results, with the 2:1 mixture of Pt:Pd providingconversion equal to or greater than the reference with only Pt as thePGM, with the 2:1 mixture of Pt:Pd providing conversion equal to orgreater than the reference with only Pt as the PGM.

FIGS. 16-18 and 19-21 show HC conversions from the three samples from150° C. to 500° C. in fresh and aged samples, respectively. Again, freshand aged samples provided similar results. The reference samples, whichonly contain Pt as the PGM, had a maximum HC conversion of about 70% atabout 450° C. to 500° C. However, both samples containing mixtures of Ptand Pd provided about 70% HC conversion by about 375° C. and 90% orgreater HC conversion by 500° C. This demonstrates that a mixture of Ptand Pd in an ASC is able to oxidize much more hydrocarbons than from theuse of Pt alone.

Example 2

Samples of catalysts prepared as describe in Example 1 were placed in anexhaust system along with a diesel oxidation catalyst (DOC) and acatalysed soot filter (CSF). The catalysts were placed in the exhaustsystem in the order SCR:ASC:DOC:CSF. The exhaust system was connected toengine and urea was injected into the exhaust stream before the SCRcatalyst. The outlet from a fuel injector was also located in the systembefore the SCR. The system was conditioned by running the engine for 1hour at 450° C., and then the engine speed was reduced to allow theengine temperature to stabilize at about 300° C. After the temperaturestabilized, fuel was injected into the exhaust system before the SCRcatalyst to raise the temperature after the CSF to about 450° C. Aftermaintaining the temperature after the CSF constant for about 15 minutes,the addition of fuel into the exhaust system was stopped and thetemperature was allow to return to about 300° C.

The temperatures at the inlet to the SCR, and the outlets from the ASC,DOC and CSF were measured. These temperature are shown in FIG. 22-24 forthe reference contain Pt only, the catalyst with 18 g/ft³ of PGM with a1:5 Pt:Pd loading, and the catalyst with 18 g/ft³ of PGM with a 2:1Pt:Pd loading, respectively. FIG. 22 shows that the use of a catalystcontaining only Pt as the PGM resulted in the outlet temperature at theASC reaching a maximum of about 350° C. at about 1250 sec, then droppingto about 300° C. by about 1600 sec. However, in both of the system usinga combination of Pt and Pd, the outlet temperature in at the ASC reacheda maximum of about 390° C. at about 1250 sec and the temperatureremained at about 390° C. by until about 2200 sec, indicating that bothof these catalysts generated an exotherm. This stable exotherm was notobserved in the reference catalyst containing only Pt as the PGM. Thisdemonstrates that when using an SCR before an ASC, the SCR did notgenerate a stable exotherm. These results also show that the catalystarticles described herein are able to provide better exotherms thanprovided by the reference system.

The generation of an exotherm can heat the catalyst to a temperaturewhere a catalyst, such as a Cu SCR catalyst, can undergo sulphurregenerated or where the SCR reaction efficiency is increased, leadingto better performance in low load operations or in cold starts. When theASC is first in the system, the generation of an exotherm can allow thecatalyst to regenerate itself.

The presence of a PGM in the catalyst will reduce the risk of highlyexothermal reactions in the catalytic system caused by the oxidation ofhydrocarbons or other reactive species that can accumulate on a catalystand be released and combusted during events that result in increasedexhaust gas temperatures.

Example 3—Creation of an Exotherm on the ASC

An ammonia slip catalyst described above is placed first in an exhaustafter-treatment system. In a conventional system, an SCR catalyst wouldbe placed in this position, as shown in Example 2. Alternatively, theammonia slip catalyst is placed in a position such that an exotherm isnot be generated upstream of the ammonia slip catalyst.

The generation of an exotherm can heat the catalyst to a temperaturewhere a catalyst, such as a Cu SCR catalyst, can undergo sulphurregenerated or where the SCR reaction efficiency is increased, leadingto better performance in low load operations or in cold starts. When theASC is first in the system, the generation of an exotherm can allow thecatalyst to regenerate itself.

The presence of a PGM in the catalyst will reduce the risk of highlyexothermal reactions in the catalytic system caused by the oxidation ofhydrocarbons or other reactive species that can accumulate on a catalystand be released and combusted during events that result in increasedexhaust gas temperatures. By having a platinum group metal incorporatedinto the catalyst, an exotherm can be generated by reacting hydrocarbonsinjected either directly into the exhaust line or coming from the engineonto the ASC itself. The exotherm generated can heat up the catalyst toa temperature where a catalyst, such as a Cu SCR catalyst, can undergosulphur regenerated or where the SCR reaction efficiency is increasedleading to better performance in low load operations or in cold starts.The use of Pd as a PGM is particularly useful because of its goodperformance in exotherm generation and relatively poor ammonia oxidationproperties.

Example 4: System Less Sensitive to HC Storage

The ammonia slip catalyst is placed either first in the exhaustafter-treatment system or in a position that large amounts ofhydrocarbons reach the ammonia slip catalyst. The invention is comparedwith a conventional system design where an SCR catalyst would be placedin the same position instead.

The storage of large amounts of hydrocarbons, combined with a fastrelease of the hydrocarbons as the system heats up, can result in alarge exotherm that can result in very high temperatures in thecatalyst. These very high can deactivate the catalyst. In some cases,the large exotherm produced can even melt the catalyst. The presence ofa PGM can reduce the risk of the formation of a very large exotherm byburning off the hydrocarbons before they can be present at a level tocause the very high exotherm. By having a platinum group metalincorporated into the catalyst, many of the hydrocarbons reaching thecatalyst will be oxidized rather than becoming trapped within an SCRcatalyst framework, as commonly occurs. This will reduce the risk forrunaway exothermal events where trapped hydrocarbons react and suddenlycause very high temperatures that deactivate exhaust gas after-treatmentsystems.

Example 5: System with Very Good Slip Control

The ammonia slip catalyst is placed after the injection point of urea(or other NH₃ source). In a conventional system, an SCR catalyst wouldbe placed in this position.

By placing the ASC very early in the system, the control of the ammoniainjection will be easier because of the reduced risk for ammonia slip.This is especially useful if the size of the catalyst is very limited ascan be the case if the ammonia slip catalyst is placed upstream of aconventional SCRT or CCRT system. This system can reduce N₂O formation.Ammonia can react on a DOC or CSF to form N₂O. By minimizing ammoniaslip, the amount of N₂O that can be formed by reaction of ammonia on aDOC or a CSF is also reduced.

The benefits of this system are that: (1) over dosage of NH₃ ispossible, (2) a less stringent dosing strategy is needed, and (3) thereis less formation of N₂O on downstream components by slipping NH₃ (onDOC or CSF).

Example 6: Incorporation of the PGM into the SCR Catalyst

For each of the examples above, the ASC is a single layer catalyst thatis a mixture of the PGM with the SCR catalyst rather than a bi-layercatalyst with a bottom layer comprising a platinum group metal and a toplayer comprising a first SCR catalyst.

The use of the mixture of the two catalysts reduces the backpressure inthe system as well as reducing production cost. Preferably, the ammoniaslip catalyst comprises a blend of platinum on a support with lowammonia storage and SCR catalyst.

The catalyst article described herein are especially useful during coldstart, where the temperature is lower than that generally needed for thePGM to oxidize ammonia and reduce the SCR activity of the system. Forexample, Pd oxidation of ammonia generally does not become importantuntil a temperature of about 400° C., with is above typical cold starttemperatures.

The preceding examples are intended only as illustrations; the followingclaims define the scope of the invention.

We claim:
 1. A method of treating an exhaust gas, comprising contactingthe exhaust gas with a catalyst article comprising a substratecomprising an inlet end and an outlet end, a first zone and a secondzone, where the first zone comprises an ammonia slip catalyst (ASC)comprising a platinum group metal and a first SCR catalyst comprising ametal exchange molecular sieve, vanadium or a base metal, and the secondzone comprises a second catalyst selected from the group consisting of adiesel oxidation catalyst (DOC), a diesel exotherm catalyst (DEC), acatalyzed soot filter (CSF), a NOx absorber, a selective catalyticreduction/passive NOx adsorber (SCR/PNA), a cold-start catalyst (CSC) ora three-way catalyst (TWC), where the first zone is located upstream ofthe gas flow relative to the second zone.
 2. The method of claim 1,where the first zone and the second zone are located on the samesubstrate and the first zone is located on the inlet side of thesubstrate and the second zone is located on the outlet side of thesubstrate.
 3. The method of claim 1, further comprising a secondsubstrate, where the first zone is located on a first substrate and thesecond zone is located on the second substrate and the first substrateis located upstream of the second substrate.
 4. The method of claim 1,where the ammonia slip catalyst comprises a bottom layer comprising aplatinum group metal and a top layer comprising the first SCR catalystlocated over the bottom layer.
 5. The method of claim 1, where theammonia slip catalyst comprises a blend of a platinum group metal on asupport with low ammonia storage and a first SCR catalyst.
 6. The methodof claim 1, where the second zone comprises a blend of a dieseloxidation catalyst and a second SCR catalyst.
 7. The method of claim 1,further comprising a second SCR catalyst, where the second SCR catalystis located between the ammonia slip catalyst and the second catalyst. 8.The method of claim 1, where the platinum group metal comprisesplatinum, palladium or a combination of platinum and palladium.
 9. Themethod of claim 1, where the platinum group metal is on a support withlow ammonia storage.
 10. The method of claim 1, where platinum ispresent from at least one of: (a) 0.01-0.3 wt. %, (b) 0.03-0.2 wt. %,(c) 0.05-0.17 wt. %, and (d) 0.07-0.15 wt. %, inclusive, relative to theweight of a support of platinum+the weight of platinum+the weight of thefirst SCR catalyst in the blend.
 11. The method of claim 7, where thesecond SCR catalyst is a base metal, an oxide of a base metal, amolecular sieve, a metal exchanged molecular sieve or a mixture thereof.12. The method of claim 11, where the molecular sieve or the metalexchanged molecular sieve is small pore, medium pore, large pore or amixture thereof.
 13. The method of claim 1, where the article comprisesonly one ammonia slip catalyst (ASC) and either (a) the second zone doesnot comprise a diesel oxidation catalyst (DOC) or (b) the DOC is notlocated adjacent to, and downstream of, the ammonia slip catalyst. 14.The method of claim 1, further comprising a third zone comprising an ASCcatalyst, where the third zone is located between the first zone and thesecond zone.
 15. The method of claim 14, where the PGM in the first zonecomprises palladium in an amount of about 1 g/ft³ to about 20 g/ft³. 16.An exhaust system comprising a. a catalyst article comprising asubstrate comprising an inlet end and an outlet end, a first zone and asecond zone, where the first zone comprises an ammonia slip catalyst(ASC) comprising a platinum group metal and a first SCR catalystcomprising a metal exchange molecular sieve, vanadium or a base metal,and the second zone comprises a second catalyst selected from the groupconsisting of a diesel oxidation catalyst (DOC), a diesel exothermcatalyst (DEC), a catalyzed soot filter (CSF), a NOx absorber, aselective catalytic reduction/passive NOx adsorber (SCR/PNA), acold-start catalyst (CSC) or a three-way catalyst (TWC), where the firstzone is located upstream of the gas flow relative to the second zone;and b. a means of introducing NH₃ into the exhaust gas or forming NH₃ inthe exhaust gas.
 17. The exhaust system of claim 16, further comprisinga catalysed soot filter (CSF).
 18. The exhaust system of claim 17, wherethe catalysed soot filter comprises a PGM loading from about 5 g/ft³ toabout 20 g/ft³ in the inlet side of the filter.